1. Field of the Invention
This invention generally relates to a coating composition for coating implantable devices, such as a stent.
2. Description of the Background
Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location.
Stents are used not only for mechanical intervention, but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
The glass transition temperature (Tg) of a polymer plays an important role in defining coating characteristic. A coating material with a very low Tg induces unacceptable rheological behavior upon mechanical perturbation such as crimping of the coated stent on a balloon as well as the sheer force exerted on the coated stent by balloon expansion and withdrawal. As used herein, the term “unacceptable rheological behavior” refers to the rheological behavior of the coating which is not suitable for use as a coating on a medical device such as a stent. On the other hand, a coating material with a high Tg introduces brittle fracture in the high strain areas of the coating. FIG. 1 is a scanning electron micrograph of a poly(ester amide) benzyl ester coated small Vision stent (12 mm, available from Guidant Crop.) depicting the typical degree of damage observed post-expansion. Accordingly, the glass transition temperature of the polymer needs to be adjusted so as to provide suitable coating characteristics such that the coating is not too soft or too brittle. The adjustment in the glass transition temperature should be aimed at eliminating or significantly reducing coating damage caused by stent expansion, crimping or damage caused by the balloon.
The biocompatibility of the polymer, moreover, plays an important role in drug delivery stent technology. The coating should not provide for an adverse response so as to negate the affects of the drug on the stented lesion. Should the coating cause, for example, excessive inflammation, the feasibility of using the stent, despite is medicinal effects, is compromised. Accordingly, the coating should be inert or otherwise provide for a positive biological response.
The polymer used for the coating should also provide for control over delivery of a therapeutic agent. The coating should not release the agent too fast or the agent should not remain in the coating for an unacceptable period of time. If the drug is released too quickly, it may not provide an efficacious dose for an adequate duration of time. If the drug is released too slowly, it may not provide for an acceptable treatment regiment.
The various embodiment of the present invention address these and other needs.